A fully-parallelized, highly-efficient compositional implicit hydrocarbon reservoir simulator is provided. The simulator is capable of solving giant reservoir models, of the type frequently encountered in the Middle East and elsewhere in the world, with fast turnaround time. The simulator may be implemented in a variety of computer platforms ranging from shared-memory and distributed-memory supercomputers to commercial and self-made clusters of personal computers. The performance capabilities enable analysis of reservoir models in full detail, using both fine geological characterization and detailed individual definition of the hydrocarbon components present in the reservoir fluids.

In a WAG flood oil is displaced from a subterranean formation by injecting water alternately with gas into a single injection completion per pattern. The ratio of water to gas injected is the WAG ratio. In this invention, two separate injection completions are used in each pattern, with one placed directly above the other. A very low WAG ratio is used for injection into the bottom extremity of the formation. A very high WAG ratio is injected into the upper interval, at as high a rate as can safely be used without fracturing the formation. In the preferred embodiment, two horizontal well bores serve as the two completion intervals. Proper design of this method gives a vertical sweep efficiency of the gas that is several-fold greater than the best of previous WAG flood designs, especially in thin formations.

A method having application notably to the development of petroleum reservoirs for fast generation of a geostatistical reservoir model on flexible grid, representative of a porous heterogeneous medium. A flexible grid best discretizing the medium is first generated. A regular Cartesian grid whose cell size, in a given direction, is greater than or equal to the largest size of the cell of the flexible grid in the same direction is then generated. On this Cartesian grid, at least one realization of petrophysical quantities characteristic of the heterogeneous medium is simulated from a geostatistical simulator. Then, this realization is resampled with a smaller discretization interval in order to know the petrophysical value associated with the realization at any point of the medium. Finally, the values of the simulated petrophysical properties are assigned to the center of the cells of the flexible grid.

A targeted heterogeneous medium in the form of an underground layered formation is gridded into a layered structured grid or a layered semi-unstructured grid. The structured grid can be of the irregular corner-point-geometry grid type or the simple Cartesian grid type. The semi-unstructured grid is really unstructured, formed by arbitrarily connected control-volumes derived from the dual grid of a suitable triangulation; but the connectivity pattern does not change from layer to layer. Problems with determining fluid movement and other state changes in the formation are solved by exploiting the layered structure of the medium. The techniques are particularly suited for large-scale simulation by parallel processing on a supercomputer with multiple central processing units (CPU's).

The invention is a method for simulating one or more characteristics of a multi-component, hydrocarbon-bearing formation into which a displacement fluid having at least one component is injected to displace formation hydrocarbons. The first step of the method is to equate at least part of the formation to a multiplicity of gridcells. Each gridcell is then divided into two regions, a first region representing a portion of each gridcell swept by the displacement fluid and a second region representing a portion of each gridcell essentially unswept by the displacement fluid. The distribution of components in each region is assumed to be essentially uniform. A model is constructed that is representative of fluid properties within each region, fluid flow between gridcells using principles of percolation theory, and component transport between the regions. The model is then used in a simulator to simulate one or more characteristics of the formation.